The present invention relates to mitigation of false co-channel uplink reception (also known as xe2x80x9cshow-thruxe2x80x9d) in a satellite communication system. In particular, the present invention relates to the application of cover sequences at earth terminals that allows a receiving satellite to reliably discard show-thru signals.
A multi-beam processing satellite system may transmit and receive numerous beams distinguished by the frequency and polarization of the signals present in the beam. The same frequency, same polarization beams (i.e., the same xe2x80x9ccolorxe2x80x9d beams) may be reused in an antenna pattern to provide communications services across a coverage area often hundreds of miles in diameter.
Beam reuse, however, renders the same color beams susceptible to co-channel interference such that a portion of the signal from a beam B is observed in a beam A. Such co-channel interference (CCI) arises primarily because of the practical limitations in implementing an antenna system which, ideally, would provide perfect rejection of same-color signals from other than the desired beam. In practice, however, the coverage provided for beam A inevitably provides some response to signals originating in beam B due to real world limitations in the physical realization of the antenna system. Furthermore, in typical frequency reuse systems, 12 to 16, or more beams of the same color may be reused over a coverage area, correspondingly increasing the potential for CCI.
Processing satellite systems may further employ TDMA (time division multiple access) techniques that permit several user earth terminals to time share a frequency channel by sending short traffic bursts in assigned time slots. These bursts typically exploit powerful error correcting codes to ensure the integrity of the traffic being carried. However, when the time slots in two same-color beams, A and B, are time aligned, a phenomenon known as show-thru may occur.
Specifically, show-thru may occur in beam A when no burst is present in a slot in beam A but a burst is present in the matching channel and slot in beam B. As a result of CCI, the burst from beam B couples (albeit in attenuated form) into the receiving electronics for beam A. Although the signal to noise ratio of this inadvertently coupled signal may be quite low, the error correcting code applied to the burst in beam B improves the likelihood that the burst from B will be regarded as valid by the processor for beam A.
In systems intended for use with the ATM (Asynchronous Transfer Mode) protocol, the inadvertent presence of apparently valid, but actually misinserted cells (due to show-thru bursts) is deleterious since the cell misinsertion rate (CMR) must be kept very low to minimize the potential for confusion at higher layers of the ATM communications protocol. Furthermore, show-thru presents a security threat to the information in the uplink burst for beam B. In other words, neighboring quiescent channels may actually decode (and ultimately send to unintended recipients) an uplink burst intended for a completely different receiver. Show-thru thus undesirably and unnecessarily utilizes satellite resources, permits unauthorized usage of the uplink beam, and compromises the security in the transmitted data.
Therefore, a need is present in the industry for an improved satellite communications system which overcomes the disadvantages discussed above and previously experienced.
It is an object of the present invention to greatly reduce the likelihood of successfully decoding show-thru signals.
Another object of the present invention is to provide a simple and effective method of reliably discarding show-thru signals at a receiver.
It is another object of the present invention to greatly reduce the likelihood of successfully decoding show-thru signals without adding overhead to uplink bursts.
One or more of the foregoing objects is met in whole or in part by a method for preventing show-thru among uplink beams transmitted to a satellite. The method includes the steps of selecting a scrambling sequence A and selecting a scrambling sequence B differing from the scrambling sequence A. The method then applies the scrambling sequence A to uplink A data to form scrambled uplink A data and further applies the scrambling sequence B to uplink B data to form scrambled uplink B data. The uplink A and the uplink B are thereby differentiated and have vastly improved resistance to show-thru. The scrambled uplink A data and the scrambled uplink B data are then transmitted to a receiver, for example, onboard a satellite.
The method also codes the uplink A data and the uplink B data prior to scrambling. The coding, in general, applies an error correcting code capable of correcting T errors. Preferably, the scrambling sequence B differs from the scrambling sequence A in more than T locations to prevent a decoder for uplink A from correcting errors generated in the decoder by scrambling sequence B.
The method is generally applicable to numerous same color uplink beams in a coverage area. Thus, the method may operate on additional uplink beams, for example, an uplink beam C. As with uplink beams A and B, the method selects a scrambling sequence C differing from the other scrambling sequences. The uplink C data is then coded and the scrambling sequence C is applied. The resultant uplink beam C thereby benefits from increased resistance to show-thru from uplink beams A and B, uplink B resists show-thru from uplink beams A and C, and uplink A resists show-thru from uplink beams B and C.
In applying the scrambling sequences to the uplink data, the method may use an exclusive-or operation. In applying the coding, the method may use a Reed-Solomon code, and in particular a (236, 212) Reed-Solomon code (T=12) particularly suitable for 53-byte Asynchronous Transfer Mode (ATM) cells. No extra uplink data is added with the exclusive-or operation and only a minimal additional processing step is used before transmission. Furthermore, the method is compatible with established error correcting decoding techniques, for example, Reed-Solomon decoding.